1. Field of the Invention
The present invention relates to a reception circuit for a cellular telephone which shares the use of both a code division multiple access system and a frequency division multiple access system, and more particularly to a peripheral circuit including a low noise amplifier circuit in a reception circuit.
2. Description of the Related Art
With reference to FIG. 3, the description will be made of a part of a conventional reception circuit for so-called dual-mode cellular telephone, for which two systems: code division multiple access system and frequency division multiple access system can be used.
In FIG. 3, the cellular telephone is capable of transmitting to and receiving from a base station by either the code division multiple access system (hereinafter, referred to as CDMA mode), or the frequency division multiple access system (hereinafter, referred to as FM mode). A transmission signal from a transmission circuit 41 is transmitted from an antenna 43 toward a base station (not shown) through an antenna multiplexer 42, while a transmission signal from a base station (not shown) is received by the antenna 43, and is inputted to a reception circuit 44 through the antenna multiplexer 42.
A transmission signal (received signal of the cellular telephone) from a base station (not shown) has a frequency of a band of 880 MHz, and within this band, a frequency for use is allocated according to the respective mode. Also, a band width occupied by one call channel is nearly 1.23 MHz in the CDMA mode, and nearly 30 KHz in the FM mode. Transmission signals in these two modes are always inputted to the antenna 43 in a mixed state.
Next, the reception circuit 44 will be described below. A received signal of a nearly 880 MHz band inputted through the antenna multiplexer 42 is amplified by a low noise amplifier circuit 45, thereafter passes through a SAW filter 46, and is inputted to a mixer circuit 47, whereby they are converted into an intermediate frequency of nearly 85 MHz. After being amplified by next intermediate frequency amplifier circuits 48 and 49 (two-stage structure is shown here), a sound signal is fetched by a demodulator circuit (not shown) and the like, connected to the latter stages of these intermediate frequency amplifier circuits 48 and 49.
The intermediate frequency signal is detected by the demodulator circuit (not shown) connected to the latter stages of the intermediate frequency amplifier circuits 48 and 49, and this detected voltage is supplied to the intermediate frequency amplifier circuits 48 and 49 through an AGC voltage terminal 50 as ACC voltage. In this respect, between the antenna multiplexer 42 and the low noise amplifier circuit 45, there is provided an impedance matching circuit 53 consisting of inductors 51 and 52 which are connected to each other in a L-character shape so as to match output impedance of the antenna multiplexer 42 and input impedance of the low noise amplifier circuit 45.
The low noise amplifier circuit 45 is constituted by amplification elements such as bipolar transistor 54, fixed bias voltage is applied to its base, and a common-emitter type is used. A received signal amplified by the transistor 54 is fetched from the collector of the transistor 54 through a coupling capacitor 55 and is inputted to the SAW filter 46.
In the cellular telephone system, a signal level transmitted from a base station to a cellular telephone, which is a mobile station, is always constant. For this reason, if the cellular telephone is far away from the base station, the received signal level becomes low, but the received signal level becomes high in the vicinity of the base station to the contrary. The level of a signal transmitted from the base station is maintained on a sufficient level so that it can be received even by a cellular telephone at a distant place. Accordingly, the reception circuit 44 for a cellular telephone has a great dynamic range (nearly 80 dB) so as to receive a low-level signal to a high-level signal without distortion. To this end, the reception circuit 44 controls the gain of the intermediate frequency amplifier circuit 48 and 49 constituted by a predetermined number of stages with AGC voltage so as to withstand this great dynamic range.
On the other hand, in order that the level of a signal transmitted from a cellular telephone toward a base station may become constant on the base station side, the transmitted signal level is made high if the cellular telephone is far away from the base station, and is made low in the vicinity of the base station. For this reason, the transmission circuit 41 has also a great dynamic range (nearly 80 dB) so as to transmit a low-level signal to a high-level signal.
As described above, a signal transmitted from the base station is transmitted at such a level as to be receivable even by a cellular telephone at a distant place. Therefore, for example, if the cellular telephone is used in the vicinity of the base station, an exceedingly-high level signal will be received, and accordingly, the gain of the intermediate frequency amplifier circuits 48 and 49 is reduced through AGC voltage from the ACC voltage terminal 50, the level of a signal inputted to a demodulator circuit and the like at the latter stage (not shown) is made constant, and particularly the intermediate frequency amplifier circuit 49 at the second stage and the subsequent circuits are adapted to cause no distortion.
If, however, a specified cellular telephone is used in a distant area from a base station, the reception level of a transmitted signal (hereinafter, referred to as a desired signal) from the base station becomes low, and therefore, AGC voltage, at which these intermediate frequency amplifier circuits 48 and 49 have the maximum gain, is supplied to these intermediate frequency amplifier circuits 48 and 49, resulting in that the intermediate frequency amplifier circuits 48 and 49 enter an activated state with the maximum gain. If, in such a state, there exists another cellular telephone transmitting to and receiving from the base station in a near district, this cellular telephone is transmitting a high-level signal (hereinafter, referred to as non-desired signal) and therefore, a high-level non-desired signal from another cellular telephone will be inputted to the reception circuit 44 through the antenna 43 of the specified cellular telephone.
Therefore, the low noise amplifier circuit 45 amplifies both the desired signal and the non-desired signal at the same time, and yet the level of the non-desired signal is high. Accordingly, there arises a problem that great distortion due to mutual modulation will occur in this low noise amplifier circuit 45. Also, since the non-desired signal amplified by the low noise amplifier circuit 45 is inputted to a mixer circuit 47 through the SAW filter 46, great distortion due to mutual modulation occurs also in this mixer circuit 47.
Further, since the specified cellular telephone generates AGC voltage on the basis of the desired signal from the base station as described above, the intermediate frequency amplifier circuits 48 and 49 operate in a maximum gain state. To that end, the received signal amplified by the low noise amplifier circuit 45 and inputted through the mixer circuit 47 will be further amplified by the intermediate frequency amplifier circuits 48 and 49. Therefore, particularly, the operation of the intermediate frequency amplifier circuit 49 at the latter stage will be saturated to cause more and more great distortion, thus resulting in call impossibility in the worst case. That is, the receive interference will occur due to the non-desired signal from the another cellular telephone.
Particularly when this specified cellular telephone is used in the CDMA mode, receive interference is more prone to occur than when it is used in the FM mode. As one of the reasons, it is considered that the band width of one call channel is large (1.23 MHz) in the CDMA mode and is small (30 KHz) in the FM mode. In other words, because when the band width of one call channel is large, it is probable that the frequency of mutual modulation distortion caused by the existence of a non-desired signal from another cellular telephone is positioned within the band of this call channel.
Thus, even if in the vicinity of a cellular telephone in use, there may exist another cellular telephone, which is transmitting a high-level signal, the present invention is to prevent receive interference from occurring.